Scale control composition for high scaling environments

ABSTRACT

A barium sulfate and/or calcium carbonate scale inhibitor composition is presented composed of a water-soluble polymer having incorporated phosphate functionality, the polymer being formed from at least one ethylenically unsaturated carboxylic acid monomer, at least one ethylenically unsaturated vinyl sulphonate monomer, or a mixture thereof. The scale inhibitor composition can also be used as a means of detecting the inhibitor composition in downhole and topside oilfield treatments by means that are faster and more accurate than turbidometric measurement.

[0001] The present invention relates to a barium sulfate scale inhibitorcomposition containing a water-soluble carboxylic acid or sulphonatepolymer having phosphate functionality. The invention also relates to amethod for reducing calcium carbonate and/or barium sulfate scale inhigh scaling environments, especially in subterranean oil fields.Additionally the scale inhibitor can be detected by inductively coupledplasma-atomic emission spectroscopy (ICP) or UV-vis, providing a methodfor measuring the concentration of inhibitor in both downhole andtopside treatments.

BACKGROUND OF THE INVENTION

[0002] Subterranean oil recovery operations can involve the injection ofan aqueous solution into the oil formation to help move the oil throughthe formation and to maintain the pressure in the reservoir as fluidsare being removed. The injected water, either surface water (lake orriver) or seawater (for operations offshore) contains soluble salts suchas sulfates and carbonates. These salts may be incompatible with theions already contained in the oil-containing reservoir (formationwater). The formation water may contain high concentrations of certainions that are encountered at much lower levels in normal surface water,such as strontium, barium, zinc and calcium. Partially soluble inorganicsalts, such as barium sulfate and calcium carbonate, often precipitatefrom the production water as conditions affecting solubility, such astemperature and pressure, change within the producing well bores andtopsides. This is especially prevalent when incompatible waters areencountered such as formation water, seawater, or produced water.

[0003] Barium sulfate and strontium sulfate form very hard, veryinsoluble scales that are difficult to prevent. Barium and strontiumsulfates are often co-precipitated with radium sulfate, making the scalemildly radioactive and introduces handling difficulties. Unlike commoncalcium salts, which have inverse solubility, barium (strontium andradium) sulfate solubility is lowest at low temperature, and this isparticularly problematic in processing oil where the temperature of thefluids decreases. Modern extraction techniques often mean that thetemperature of the produced fluids (water, oil and gasmixtures/emulsions) are decreased (as low as 5C) and contained inproduction tubing for long periods (24 hrs or longer). Calcium carbonatecan be readily removed using HCl acid washing should scale occur. Thiscan be performed topside or downhole, is cheap, and is non-invasive.Dissolution of sulfate scales is difficult (requiring high pH, longcontact times, heat and circulation) and can only be performed topside.Alternatively, milling and in some cases high-pressure water washing canbe used. These are expensive, invasive procedures and require processshutdown. Inhibition is the key approach to sulfate scales, especiallydownhole.

[0004] Barium sulfate, or other inorganic supersaturated salts, canprecipitate onto the formation to form a scale, thereby clogging theformation and restricting the recovery of oil from the reservoir. Theinsoluble salts may also precipitate onto production tubing surfaces andassociated extraction equipment that can limit productivity, limitproduction efficiency, and compromise safety. Certain oil-containingformation waters are known to contain high barium concentrations of 400ppm, and higher. Since barium sulfate forms a particularly insolublesalt, the solubility of which declines rapidly with temperature, it isdifficult to inhibit scale formation and to prevent plugging of the oilformation and topside processes and safety equipment.

[0005] While “scale inhibition” and “deposit control” are generic termswithout mechanistic implications, there are two generally acceptedmechanisms for controlling the amount of divalent metal ions fouling ordepositing in the surface of the formation: 1) inhibiting precipitationof the material from the process water, or 2) dispersing the materialonce it has formed, to prevent it from attaching to the surfaces. Theexact mechanism by which a particular scale inhibitor functions, and theinterplay between these two or other mechanisms is not well understood.The compositions of the present invention may operate by either or bothof these routes

[0006] Current methods for inhibiting barium sulfate scaling involve theuse of expensive organic phosphonic acids, as described in U.S. Pat.Nos. 6,063,289 and 6,123,869. Acrylic polymer scale inhibitorscontaining a phosphino or phosphono moiety are also used. U.S. Pat. No.4,209,348 describes a copolymer of (meth)acrylic acid having a phosphatefunctionality that is useful as a combined scale and corrosion inhibitorin industrial water treatment. This chemistry provides only limitedadhesion to the oil-containing formation. U.S. Pat. No. 4,711,725describes the use of terpolymers of (meth)acrylicacid/2-acrylamido-2-methyl propane sulfonic acid/substituted acrylamidesfor inhibiting the precipitation of calcium phosphate.

[0007] EP 459661 A1 describes a method for silica scale inhibition usinga mixture of aluminum or magnesium ions with a low molecular weightpoly(meth)acrylic acid or polymaleic acid, plus either a copolymer or aterpolymer of a) (meth)acrylic acid or maleic acid with b)(meth)acrylamido methyl propane sulfonic acid, or styrene sulfonic acid,and c) another monomer which could be a vinyl ester, and the vinyl estercould contain a phosphate group.

[0008] Surprisingly it has been found that the addition of a phosphatemoiety to a polyacrylate or polysulphonate scale inhibitor allows forgreater adsorption to an oilfield reservoir, thus allowing for anincrease in the treatment lifetime, while still retaining good scaleinhibition properties. Polymeric inhibitors also have the advantage ofbeing relatively unmetabolized, and therefore have low toxicity andbioaccumulation characteristics.

SUMMARY OF THE INVENTION

[0009] The present invention is directed to a scale inhibitorcomposition for barium sulfate scale and for calcium carbonate scale,comprising a water-soluble polymer having phosphate functionality,wherein said polymer is selected from the group consisting of

[0010] a) a polymer formed from at least one ethylenically unsaturated(di)carboxylic acid monomer,

[0011] b) a polymer formed from at least one ethylenically unsaturatedsulphonate monomer, and

[0012] c) a mixture thereof.

[0013] Other embodiments of the invention are methods for inhibiting theformation of barium sulfate scale and calcium carbonate scale,comprising forming the inhibitor composition of the invention, andcontacting said inhibitor composition with a surface in contact with anaqueous solution containing barium and sulfate ions.

[0014] Still another embodiment of the invention is a method fordetecting the concentration of an inhibitor solution for use insubterranean oil field use comprising forming the inhibitor compositionof the invention; injecting said inhibitor composition into asubterranean oil-containing formation; bringing a aqueous solutioncontaining the inhibitor composition from the subterraneanoil-containing formation to a location above the oil-containingformation, and analyzing for the phosphate moiety.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention is directed to a scale inhibitor for bariumsulfate scale and calcium carbonate scale comprising a water-solublepolymer polymerized from at least one ethylenically unsaturatedcarboxylic acid monomer or sulphonate monomer, where the polymercontains a phosphate functionality. Properties desirable in a bariumscale inhibitor for use in oilfield applications include that theinhibitor should a) have a high salt (especially calcium) tolerance, b)adsorb onto the oil-containing formation from a 5-30 percent activesolution, c) not desorb under high shear, d) be water-soluble and shoulddesorb at a concentration above the minimum inhibitor concentration(MIC) for as long a period as possible, and e) be effective under thehigh-temperature and high-pressure environments encountered insubterranean oil field applications, as well as lower pressure and lowertemperature environments that might be experienced in the process ofseparating the oil, gas and water.

[0016] The scale inhibitor of the present invention is a low molecularweight water-soluble polymer based on a (di)carboxylic acid and/orsulphonate monomers. A (di)carboxylic acid monomer, as used herein,refers to mono-carboxylic acid monomers, di-carboxylic acid monomers,and mixtures thereof. The carboxylic acid polymer is formed from one ormore ethylenically unsaturated carboxylic acid monomers including, butare not limited to, acrylic acid, methacrylic acid, ethacrylic acid,alpha-chloro-acrylic acid, alpha-cyano acrylic acid,alpha-chloro-acrylic acid, alpha-cyano acrylic acid, beta methyl-acrylicacid (crotonic acid), alpha-phenyl acrylic acid, beta-acryloxy propionicacid, sorbic acid, alpha-chloro sorbic acid, angelic acid, cinnamicacid, p-chloro cinnamic acid, beta-styryl acrylic acid(1-carboxy-4-phenyl butadiene-1,3), itaconic acid, maleic acid,citraconic acid, mesaconic acid, glutaconic acid, aconitic acid, fumaricacid, and tricarboxy ethylene. Preferred monomers include (meth)acrylicacid and/or maleic acid (or anhydride) polymer. The carboxylic acidpolymer may optionally include other ethylenically unsaturated monomers,as known in the art, provided the polymer contain 50 to 99.5 molepercent of one or more carboxylic acids, and at least 0.5 mole percentof a containing phosphate moiety, based on the total moles of monomer.Phosphate-functional carboxylic acid polymers are effective atinhibiting calcium carbonate scale as well as barium sulfate scale.

[0017] Sulfonate polymers are formed from at least one unsaturatedsulfonic acid monomer, including but not limited to (meth)acrylamidomethylpropane sulfonic acid, styrene sulfonic acid, vinyl sulfonic acid,3-sulfopropyl (meth)acrylate, (meth)allyl sulfonic acids, (meth)allyloxybenzene sulfonic acids, allyloxy hydroxyalkyl sulfonic acids. Preferablythe polymer includes vinyl sulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, or a mixture thereof. The sulphonate polymer containsfrom 50 to 99.5 mole percent of at least one sulfonic acid monomer, from1 to 50 mole percent of at least one other ethylenically unsaturatedmonomer, and at least 0.5 mole percent of a containing phosphate moiety,based on the total moles of monomer.

[0018] The scale inhibitor polymer preferably is a polymer formed fromat least one carboxylic acid monomer and at least one ethylenicallyunsaturated sulfonic acid monomer. The polymer may also contain otherethylenically unsaturated monomers known in the art. The incorporationof sulfate monomer into the polymer aids in stabilizing the polymer inenvironments containing high salt and high calcium concentrations. Thecarboxylic acid monomer is preferably present in the polymer at from 50to 99 mole percent. The sulfonic acid monomer is preferably present inthe polymer at from 0.5 to 50 mole percent, preferably 1 to 35 molepercent, and most preferably 2 to 20 mole percent, based on the totalnumber of moles of monomer.

[0019] The phosphate functionality provides the polymeric inhibitor withgood adsorption/desorption characteristics allowing the polymer to beretained in the reservoir and providing extended treatment lifetimes.Polymers in general exhibit poor adsorption ability, yet it has beenfound that the addition of a phosphate functionality enhancesadsorption. This makes the polymeric inhibitor of the present inventionuseful in downhole applications. The phosphate functionality can beattached to the polymer by any means known in the art, including, butnot limited to, a copolymerization, a two-stage polymerization,grafting, or attachment of a phosphate surfactant. The preferred methodfor incorporation of phosphate functionality into the inhibitor is by acopolymerization of the carboxylic acid and/or sulphonate monomer(s)with one or more phosphate-containing monomer(s), and optionally otherethylenically unsaturated monomers. Examples of phosphate-containingmonomers useful in the present invention include, but are not limitedto, phosphate (meth)acrylate monomers and (meth)allyl hydroxyphosphates. A preferred monomer is ethylene glycol methacrylatephosphate. The advantage of using a phosphate-containing monomer, overthe use of a phosphate ester surfactant, is that it allows theincorporation of a higher level of the phosphate functionality. Thephosphate monomer is incorporated at from 0.5 to 50 mole percent,preferably 1 to 35 mole percent, and most preferably 2 to 20 molepercent, based on the total moles of monomer.

[0020] The phosphate functionality may also be incorporated into thepolymer by means of a phosphate-containing surfactant, such as forexample an oleyl ethoxylated phosphate ester. The polymer is polymerizedin the presence of the phosphate surfactant, or a mixture of phosphateand other surfactants. The phosphate-containing surfactant isincorporated onto the polymeric inhibitor at a level of from 0.1 to 20mole percent, preferably 1 to 10 mole percent, based on the total molesof monomer.

[0021] Additional ethylenically unsaturated monomers, as known in theart, may also be incorporated into the polymeric scale inhibitor. Theadditional monomers may be present in the polymer at from 5 to 30 molepercent based on the total number of moles of monomer.

[0022] The preferred polymeric scale inhibitor composition of thepresent invention is a polymer of acrylic acid/2-acrylamido-2-methylpropane sulfonic acid/ethylene glycol methacrylate phosphate.

[0023] Polymerization of the polymeric scale inhibitor can be by anymeans known in the art, and by batch, semi-batch, staged, or continuouspolymerization.

[0024] The weight average molecular weight of the polymeric scaleinhibitor is from 500 to 50,000, and preferably from 2,000 to 20,000,based on a polyacrylate standard.

[0025] The phosphate-functional scale inhibitor is useful for inhibitingbarium sulfate and strontium sulfate scaling in oil-field applications,and also for calcium carbonate inhibition. The inhibitor is generallyblended into the downhole treating solution at a level of from 1 to 500ppm, preferably 10 to 150 ppm, based on the total solution. In additionto the inhibitor, the process solution is generally a mixture of freshand/or salt water to which has been added other additives such asanti-corrosion agents, biocides and others chemicals as appropriate totreat the well conditions.

[0026] The scale inhibitor is applied to the reservoir in a processcalled a squeeze. The squeeze is a three-stage process by which fluidsare injected directly into the wellbore, reversing the flow of liquidback down into the reservoir. First a dilute solution of scale inhibitor(0.1%) with surfactant (0.1%) is applied to clean and cool the nearwellbore. This step is followed by a high concentration solution of thescale inhibitor active (called the pill) at between 5 and 20%, finallyfollowed by a low concentration solution of inhibitor which is appliedto move the pill away from the near wellbore, radially outward to adistance into the near wellbore which is designed to give maximumsqueeze life (based on laboratory modeling). The solutions are left incontact with the reservoir for between 6 and 24 hours ideally to allowfor adsorption equilibration, after which the well is returned toproduction. Adhesion to the formation allows the inhibitor to remainwithin the near-wellbore area without being pumped up in the oil/wateremulsion, returning only at concentrations in the aqueous phase below250 ppm (and above MIC) for an extended period thus providing a longertreatment lifetime. The phosphate moiety on the polyacrylate allows forthe combination of nucleation inhibition with crystal growth retardationand crystal growth modification. The addition of a sulfonic acid monomerto the polymer also allows excellent compatibility with the formationwater (especially high calcium brines) and is known to give greaterefficacy when encountering barium scales. Although squeeze applicationof the chemical is the most common method of treating downhole scale,the product could also be applied by other techniques commonly usedoffshore which include: gas-lift injection, downhole annulus injection,encapsulation or soluble matrix techniques, sub-sea wellhead injectionvia umbilical or indeed secondary topside treatments to enhanceinhibitor performance as process conditions vary scaling tendency.

[0027] One further advantage of using the composition of the presentinvention in the inhibition of oilfield scales is that for both downholeand topside treatments, the incorporation of the phosphate functionalityinto the polymer backbone provides a means to improve the detectabilityof the inhibitor. Polymers containing a phosphorus functionality can bereadily detected by ICP analysis, with a residual accuracy of less than1 ppm. For detectability purposes, the phosphate monomer is incorporatedinto the polymer as a “tag” at from 0.5 to 5 mole percent, andpreferably from 1 to 2 mole percent. This would be an alternative to thephosphinate tag technology currently used. The inclusion of thephosphorous containing monomer allows detectability for topsideinhibitors as well as for downhole types. The use of the phosphatemonomer tag provides a high degree of delectability, and is moreaccurate and quicker than the turbidometric test currently used. Priorto application of the product, experiments are conducted in a laboratoryto determine an effective minimum inhibitor concentration (MIC) whichjust inhibits inorganic scale formation under simulated productionconditions. The ability of the operator to quickly and accuratelydetermine the amount of scale inhibitor in the produced fluids andcompare this to the MIC values generated allows him to decide when it ismost suitable to retreat the reservoir or increase the topside additionrate to ensure that no damage occurs to his reservoir or equipment dueto inorganic scale deposition.

[0028] The following examples are presented to further illustrate andexplain the present invention and should not be taken as limiting in anyregard.

EXAMPLE 1 Acetate Buffered Static Barium Sulfate Inhibition EfficiencyTest

[0029] The following test was used to determine the static bariumsulfate inhibition efficiency:

[0030] 1. Prepare two brine solutions by dissolving the appropriatesalts in distilled water. Formation water (FW) Sea Water, (SW) Ion ppmppm Sodium 31,275 10,890 Calcium 5,038 428 Potassium 654 460 Magnesium739 1,368 Barium 269 0 Strontium 71 0 Sulfate 0 2,960

[0031] 2. Filter the brines through 0.45 μm membrane filters.

[0032] 3. Dissolve the scale inhibitor (SI) in the filtered seawater(SW) to 10000 ppm (as active SI). Filter this solution through 0.45 μmmembrane filter.

[0033] 4. The inhibitor solution is then diluted further into SW to givethe required concentration for the particular test and each inhibitorconcentration is tested in duplicate. (Note: the concentration ofinhibitor in each seawater solution must be higher than that requiredfor the test by a factor which accounts for the dilution when mixed withthe formation water.)

[0034] 5. Pour the appropriate volume (50 ml) of inhibitor/seawatersolution into 150 ml high-density polyethylene (HDPE) bottles.

[0035] 6. Pour the appropriate volume (50 ml) of formation water into150 ml HDPE bottles so as to give 100 mls when mixed in the requiredratio (1:1).

[0036] 7. Add 1 ml (1 ml buffer/100 ml final brine mixture) of buffersolution to the brine containing the inhibitor, taking extreme care notto introduce impurities and cap all bottles securely. The buffersolution is an acetic acid/sodium acetate buffer solution prepared inorder to give the required pH. For example in order to obtain a pH ofapproximately 5.5, the buffer solution is prepared by dissolving thefollowing amounts of Analar grade reagents into 100 mls of distilledwater: 13.50 g sodium acetate tri-hydrate+0.35 g acetic acid

[0037]  Note: *It is important to check the effectiveness of the buffersystem prior to commencement of a particular set of tests, in order toensure that the required pH is obtained following addition of the bufferto the mixed brine system. This may often lead to small modifications ofthe buffer system prior to use.

[0038] 8. Place the bottles containing the inhibitor solutions into awater bath and the bottles containing formation water (FW) in a oven atthe appropriate test temperature for 60 minutes in order to reachthermal equilibrium.

[0039] 9. Mix the two Brines together (by pouring the FW into the SW andquickly shaking.) Start a stopclock (t=0). The bottles are then replacedinto the water bath at test temperature.

[0040] 10. The tests are then sampled at the required time (t=2, 20hours) by pipetting 1 ml of the supernatant into either 9 mls or 4 mlsof 3000 ppm KCl and 1000 ppm PVS solution depending on the brine systemunder examination. Test conditions: Brine mixture: 50:50 Forties typeFW/SW Temperature: 90° C. pH: 5.5 Sampling Time: 2, and 20 hours

[0041] Sampling and Analysis: The sampling procedure is carried out asfollows: A stabilizing/dilution solution is made containing 1,000 ppmcommercial polyvinyl sulphonate scale inhibitor (PVS) and 3,000 ppmpotassium (as KCl) in distilled water. The solution of 1,000 ppm PVS hasbeen shown to effectively stabilize (or quench) the sample and thusprevent further precipitation, when used as described below. Thepotassium is included in this solution to act as an ionizationsuppressant for the Atomic Absorption determination of barium.

[0042] For these tests, either 4 or 9 ml (depending on the brine system)of the KCl/PVS stabilizing solution was pipetted into a test tube atroom temperature prior to sampling. 1 ml of the particular testsupernatant was then removed from the test bottles using an automaticpipette, taking care not to disturb any settled precipitate andimmediately added to the 4 or 9 ml of stabilizing solution. The sampleswere then analyzed by Atomic Absorption Spectroscopy (AA) for barium.

[0043] The barium sulfate inhibition efficiencies are then calculatedusing the following equation:${\% \quad E\quad {ff}\quad i\quad c\quad i\quad e\quad n\quad c\quad y_{(t)}} = {{\frac{\left( {M_{\underset{\_}{B}} - M_{\underset{\_}{I}}} \right)}{M_{\underset{\_}{B}}} \times 100} = {{\frac{\left( {C_{\underset{\_}{O}} - C_{\underset{\_}{B}}} \right) - \left( {C_{\underset{\_}{O}} - C_{\underset{\_}{I}}} \right)}{\left( {C_{\underset{\_}{O}} - C_{\underset{\_}{B}}} \right)} \times 100} = {\frac{\left( {C_{\underset{\_}{I}} - C_{\underset{\_}{B}}} \right)}{\left( {C_{\underset{\_}{O}} - C_{\underset{\_}{B}}} \right)} \times 100}}}$

[0044] Where;

[0045] M_(B)=Mass Barium precipitated in supersaturated blank solution.

[0046] M_(I)=Mass Barium precipitated in test solution.

[0047] C_(O)=Concentration of Barium originally in solution (i.e. t=0).

[0048] C_(I)=Concentration of Barium at sampling.

[0049] C_(B)=Concentration of Barium in the blank solution (noinhibitor) at the same conditions and sampling time as C_(I) above.

[0050] (t)=Sampling time.

EXAMPLE 2 Measurement of Adsorption Characteristics

[0051] 5 grams of crushed Clashach core material (between 38 micron—600micron size fraction) was mixed with 10 ml of each of each test solutioncontaining 500 ppm active inhibitor in 60 ml HDPE bottles. Clashach coreis a highly quartzitic outcrop core material with low clay content andis used as a reference material to determine relative adsorptioncharacteristics of scale inhibitor chemistries. The 500 ppm active scaleinhibitor solutions are allowed to contact the Clashach core materialfor a period of 20 hours at 95° C., after which time the test solutionsare filtered through a 0.45 micron filter and the residual scaleinhibitor measured and compared against that of the initial 500 ppmactive inhibitor solution. The scale inhibitor concentration wasmeasured using Inductively Coupled Plasma—Atomic Emission Spectroscopy(ICP-AES), which is accurate to part per billion levels. A range ofscale inhibitor standards of known concentration (0, 50, 250, 1000 forexample) are prepared and the phosphorous emission levels determined. Asthe emission is proportional to the concentration of total phosphorous,the residual inhibitor concentration from the test solution can becalculated from the standards. Due to the different level of phosphorousin each scale inhibitor, a set of standards must be run for eachdifferent polymer. Once the residual scale inhibitor concentration hasbeen determined, the adsorption of the polymer to the rock surface canbe calculated from the following equation:

Adsorption (mg/g)=(C ₁ −C ₂)/M _(R) *V

[0052] Where:

[0053] C₁=the concentration of scale inhibitor in the initial solution

[0054] C₂=the concentration of scale inhibitor left in solution afterthe test period

[0055] M_(R)=the mass of rock used in the test onto which the scaleinhibitor can adsorb

[0056] V=the volume of inhibitor solution used in the test

EXAMPLE 3 SYNTHESIS OF POLYMERIC SCALE INHIBITOR

[0057] To a 2 liter glass vessel equipped with stirrer, reflux condenserand means of temperature control; 200 g of propan-2-ol and 200 g ofdeionized water was charged then heated to reflux. A monomer mixture ofacrylic acid (200 g), 2-acrylamido-2-methyl propane sulfonic acid (141.4g) and ethylene glycol methacrylate phosphate (34.1 g) was fed over 3hours into the reactor. A initiator solution was fed concurrently withthe monomer feed but with an overlap of 30 minutes and consisted ofsodium persulfate (13.5 g), 35% hydrogen peroxide (55 g) and water (65g). When both feeds were complete the reaction was held at reflux for 30minutes then cooled. The propan-2-ol was removed by distillation on arotary evaporator. The resulting polymer was neutralized with 50 g of50% sodium hydroxide.

EXAMPLE 4 SYNTHESIS OF POLYMERIC SCALE INHIBITOR

[0058] To a 2 liter glass vessel equipped with stirrer, reflux condenserand means of temperature control; 200 g of propan-2-ol and 200 g ofdeionized water was charged then heated to reflux. A monomer mixture ofacrylic acid (200 g), 2-acrylamido-2-methyl propane sulfonic acid (141.4g) and Albritect 6835 (34.1 g product of Rhodia) was fed over 3 hoursinto the reactor. A initiator solution was fed concurrently with themonomer feed but with an overlap of 30 minutes and consisted of sodiumpersulfate (13.5 g), 35% hydrogen peroxide (55 g) and water (65 g). Whenboth feeds were complete the reaction was held at reflux for 30 minutesthen cooled. The propan-2-ol was removed by distillation on a rotaryevaporator. The resulting polymer was neutralized with 50 g of 50%sodium hydroxide.

EXAMPLE 5 SYNTHESIS OF POLYMERIC SCALE INHIBITOR

[0059] To a 2 liter glass vessel equipped with stirrer, reflux condenserand means of temperature control; 200 g of propan-2-ol and 200 g ofdeionized water was charged then heated to reflux. A monomer mixture ofacrylic acid (200 g), 2-acrylamido-2-methyl propane sulfonic acid (141.4g) and Lubrhophos LB400 (30 g oleyl ethoxylate phosphate ester fromRhodia) was fed over 3 hours into the reactor. A initiator solution wasfed concurrently with the monomer feed but with an overlap of 30 minutesand consisted of sodium persulfate (13.5 g), 35% hydrogen peroxide (55g) and water (65 g). When both feeds were complete the reaction was heldat reflux for 30 minutes then cooled. The propan-2-ol was removed bydistillation on a rotary evaporator. The resulting polymer wasneutralized with 50 g of 50% sodium hydroxide.

EXAMPLE 6 SYNTHESIS OF POLYMERIC SCALE INHIBITOR

[0060] To a 2 liter glass vessel equipped with stirrer, reflux condenserand means of temperature control; 200 g of propan-2-ol and 100 g ofdeionized water was charged then heated to reflux. A monomer mixture of25% aqueous sodium vinyl sulphonate (300 g) and Albritect 6835 (15.5 g)was fed over 3 hours into the reactor. A initiator solution was fedconcurrently with the monomer feed but with an overlap of 30 minutes andconsisted of sodium persulfate (3.6 g), 35% hydrogen peroxide (8 g) andwater (115 g). When both feeds were complete the reaction was held atreflux for 30 minutes then cooled. The propan-2-ol was removed bydistillation on a rotary evaporator. The resulting polymer wasneutralized with 10 g of 50% sodium hydroxide.

EXAMPLE 7 STATIC BARIUM SULFATE INHIBITION EFFICIENCY TEST

[0061] The polymeric inhibitors were tested in the procedure of Example1 with the following results. Diethylenetriamine pentamethylenephosphonic acid is a typical adsorbing downhole scale inhibitorfrequently employed offshore of molecular weight 564. Polyvinylsulfonate(approximate molecular weight of 5000) is an example of a typical lowadsorbing scale inhibitor. AQUATREAT® AR-545 is an acrylicacid/2-acrylamido-2-methyl propane sulfonic acid copolymer from NationalStarch and Chemical Company, having a molecular weight of about 4500.Example 3 = 15 ppm active Example 4 = 15 ppm active Example 5 = 15 ppmactive AQUATREAT ® AR-545 = 12 ppm active (comparative)polyvinylsulfonate (PVS) = 15 ppm active (comparative)diethylenetriamine = 10 ppm active pentamethylene phosphonic acid(DETPMP) (comparative)

[0062] The scale inhibitor compositions of the invention performed aswell as PVS which is commonly used to treat such scaling situations. Theperformance is not as good as that of DETPMP which is commonly used tosqueeze treat. However, the DETPMP performance is poor when thetemperature of the produced fluids decrease, whereas those of polymersof the invention improve.

EXAMPLE 8 ADSORPTION RESULTS

[0063] The scale inhibitors were tested at 500 ppm active in theadsorption test according to Example 2, with the following results.Results at pH 2 indicate adsorption mechanism through hydrogen bondingof the polymer with the reservoir substrate. Results at pH 6 indicateadsorption mechanism through calcium bridging of the polymer to thereservoir substrate. pH 2 pH 6 DETPMP 0.66 mg/g 0.52 mg/g AQUATREAT ®AR-545 0.28 mg/g 0.23 mg/g Example 4 0.28 mg/g 0.16 mg/g Example 3 0.26mg/g 0.07 mg/g Example 5 0.19 mg/g 0.22 mg/g PVS 0.10 mg/g 0.01 mg/g

[0064] In both pH scenarios, the inhibitors of the invention andAQUATREAT® AR-545 adsorbed much more strongly than the PVS polymer. Withreference to the MIC values generated, this implies that all of thecompositions of the invention will have more effective squeeze lives,due to more polymer being retained in the reservoir upon squeezing, andthe effective inhibitor dosages being similar if not better than forPVS.

What is claimed is:
 1. A scale inhibitor composition for barium sulfateand calcium carbonate scale comprising a water-soluble polymer havingincorporated phosphate functionality, wherein said polymer is formedfrom monomers selected from the group consisting of a) at least oneethylenically unsaturated (di)carboxylic acid monomer, b) at least oneethylenically unsaturated vinyl sulphonate monomer, and c) a mixturethereof.
 2. The scale inhibitor of claim 1 wherein said carboxylic acidmonomer comprises (meth)acrylic acid, maleic acid, maleic anhydride, ora mixture thereof.
 3. The scale inhibitor of claim 1 wherein said vinylsulphonate monomer comprises vinyl sulfonic acid,2-acrylamido-2-methylpropane sulfonic acid, or a mixture thereof.
 4. Thescale inhibitor of claim 1 comprising 0.5 to 50 mole percent of saidsulfonate monomer.
 5. The scale inhibitor of claim 1 comprising 0.5 to50 mole percent of said carboxylic acid monomer.
 6. The scale inhibitorof claim 1 wherein said phosphate functionality comprises from 0.5 to 50mole percent of a phosphate functional monomer incorporated into saidwater-soluble polymer, based on the total moles of monomer.
 7. The scaleinhibitor of claim 1 wherein said polymer is formed from monomerscomprising both at least one carboxylic acid monomer, and at least onesulphonate monomer.
 8. The scale inhibitor of claim 6, wherein saidphosphate functional monomer comprises at least one of ethylene glycolmethacrylate phosphate and/or oleyl ethoxylate phosphate ester.
 9. Thescale inhibitor of claim 1 wherein said polymer further is formed fromat least one other ethylenically unsaturated monomer.
 10. The scaleinhibitor of claim 1 wherein the polymer has a molecular weight of from500 to 50,000.
 11. The scale inhibitor composition of claim 1 furthercomprising barium ions, strontium ions, calcium ions, or a mixturethereof.
 12. An oil containing formation having absorbed thereon theinhibitor composition of claim
 1. 13. A method for inhibiting theformation of barium sulfate scale comprising: a) forming the inhibitorcomposition of claim 1, and b) contacting said inhibitor compositionwith a surface in contact with an aqueous solution containing barium andsulfate ions.
 14. The method of claim 13 wherein said surface is asubterranean oil-containing formation.
 15. A method for inhibitingcalcium carbonate scales in subterranean oil field use comprising: a)forming a sulphonate free inhibitor composition of claim 1; b)contacting said inhibitor composition with a subterranean oil-containingformation.
 16. A method for detecting the concentration of an inhibitorsolution for use in subterranean oil field use comprising: a) formingthe inhibitor composition of claim 1; b) injecting said inhibitorcomposition into a subterranean oil-containing formation; c) bringing aaqueous solution containing the inhibitor composition from thesubterranean oil-containing formation to a location above theoil-containing formation, and d) analyzing for the phosphate moiety. 17.The method of claim 16 wherein the analysis for the phosphate moiety isused to calculate the concentration of the inhibitor composition. 18.The method of claim 16 wherein said analyzing for the phosphate moietycomprises using either inductively coupled plasma—atomic emissionspectroscopy (ICP-AES) or a colourimetric complexation of free phosphatefrom UV degraded polymer solution
 19. The method of claim 16 whereinsaid inhibitor composition is sulphonate-free.
 20. The method of claim16 wherein the analysis for the phosphate moiety is not by aturbidometric method.